Some displays, such as liquid crystal displays (LCDs), comprise a spatial light modulator that is illuminated by a backlight. Light from the backlight interacts with the spatial light modulator which spatially modulates the light so as to present images to a viewer. The images may be still images or video images for example. The spatial light modulator may comprise an array of controllable pixels.
In some such displays, the backlight includes multiple light emitting devices, such as LEDs, for illuminating regions of the spatial light modulator. Such light emitting devices or groups of such light emitting devices may be separately controllable so that the intensity of light emitted by the backlight can be made to vary in a desired way over the spatial light modulator. Such displays are referred to herein as dual-modulation displays. Some examples of dual modulation displays are described in: U.S. Pat. No. 6,891,672 issued 10 May 2005 and entitled “High Dynamic Range Display Devices”, U.S. Pat. No. 7,403,332 issued 22 Jul. 2008 and entitled “High Dynamic Range Display Devices”, and United States Patent Application Publication No. 2008/0180466 published 31 Jul. 2008 and entitled “Rapid Image Rendering on Dual-Modulator Displays”, all of which are hereby incorporated herein by reference for all purposes.
The brightness of light emitters on a backlight may be controlled by a technique known as pulse width modulation (PWM). A light emitting device such as an LED may be switched between an ON state at 100% brightness and an OFF state at 0% brightness by switching on and off a suitable fixed electrical current through the device. PWM operates by pulsing each light emitter to its ON state for some percentage of a repeating time period. If the time period is sufficiently short (e.g. 1 millisecond) the human visual system does not detect the light emitter cycling between ON and OFF states. An observer merely perceives the average emitted light intensity, which is proportional to the percentage of the PWM period that the device is in the ON state. This percentage is referred to as the duty cycle of the PWM signal. For example, a light emitter driven by a PWM signal with a duty cycle of 75% is switched on for 75% of each PWM period and appears to an observer as if it were steadily emitting light having a brightness of 75% of its maximum brightness.